[REBOL] Re: Expression evaluator (an example of parsing blocks)
From: larry:ecotope at: 21-Oct-2000 14:35
Hi Gabriele
> How do you write A * B - C * D in REBOL without using parens? You
> need to use parens or use the notation:
You can do it without parens by mixing infix and prefix forms:
>> - 4 * 5 3 * 4
== 12
I looked for a more complex example and noted that even with parens, the
result from REBOL may be surprising, because of the way the interpreter
evaluates parens.
>> - (4 * 5) ((3 * 4) + (2 * 3))
== 18 ; not the naive expected result
>> (4 * 5) - ((3 * 4) + (2 * 3))
== 2 ; the expected result
But I found a way to do this without parens (readability becoming an issue)
>> + - + 3 * 4 2 * 3 4 * 5
== 2
This is kind of fun, eh?
Regards
-Larry
PS I suspect it can be shown that with suitable rearrangement of terms and
with both infix and prefix forms available, that parens are not needed.
Counter example?
----- Original Message -----
From: Gabriele Santilli <[g--santilli--tiscalinet--it]>
To: <[rebol-list--rebol--com]>
Sent: Saturday, October 21, 2000 4:46 AM
Subject: [REBOL] Re: Expression evaluator (an example of parsing blocks)
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